1. Field of the Invention
The present invention relates to an optical pickup device and an optical disk driver that are to be built in an electronic device, such as a personal computer, a notebook computer, and the like.
2. Description of the Related Art
FIG. 14 is a schematic diagram of an optical system in a related-art optical pickup device. A light source 201 produces outgoing light 215 oriented to an optical disk 211. An integrated prism 202 has therein oblique surfaces 203 and 204. A beam splitter 205 is made in the oblique surface 203. The beam splitter 205 separates from the outgoing light 215 reflected light 216 resulting from the outgoing light 215 undergoing reflection on the optical disk 211, and lets the reflected light 216 travel toward an optical receiver 208. An astigmatism generation element 206 made up of a Fresnel mirror 207 is formed on the oblique surface 204. The astigmatism generation element 206 produces light used in focus control. An objective lens 208 converges the outgoing light 215 on the optical disk 211. An optical receiver 209 has a light receiving section 210, and the light receiving section 210 receives reflected light 216. The optical receiver 209 converts the received light into an electric signal used in focus control, and output the electric signal.
FIG. 15A is an operation diagram of the astigmatism generation element; FIG. 15B is a view showing the geometry of a beam spot achieved when the optical disk is located at a close position; and FIG. 15C is a view showing the geometry of the beam spot achieved when the optical disk is located at a distant position. The astigmatism generation element 220 is; for instance, a cylindrical lens, and generates focal points 224 and 225 at different positions within two mutually orthogonal cross-sectional planes 222 and 223 including an optical axis 221. An optical receiver 227 is interposed between the focal point 224 and the focal point 225. Light entering along the vertical cross-sectional plane 222 passes in an unmodified state through the astigmatism generation element 220 and converges on the forward focal point 224 of the optical receiver 227, to thus enter the optical receiver 227. Meanwhile, light entering the horizontal cross-sectional plane 223 enters the optical receiver 227 so as to converge on the backward focal point 225 of the optical receiver 227, because the astigmatism generation element 220 acts as a concave lens. A spot 226 on the optical receiver 227 assumes a slightly-spread, substantially-circular geometry.
The optical receiver 227 has optical receiving sections 228A to 228D that receive light passed through the astigmatism generation element 220. The optical receiving sections 228 are arranged in the shape of a four-paned window while rotated at an angle of 45 degrees with respect to the cross-sectional planes 222 and 223. The optical receiving sections 228A and 228C are arranged in a horizontal direction, and the optical receiving sections 228B and 228D are arranged in a vertical direction. The optical receiving sections 228A to 228D convert the quantity of received light into an electric signal. Electric signals converted by the respective optical receiving sections 228A to 228D are taken as A to D. A focus error signal FES that is a signal for focus control purpose can be obtained by arithmetic operation of FES=(A+C)−(B+D).
As shown in FIG. 15B, when the optical disk is located at a close position, the focal point 224 comes close to the optical receiver 227, and the focal point 225 moves away from the optical receiver 227. Therefore, the vertical dimension of the spot 226 becomes smaller, and the horizontal dimension of the same becomes larger, whereupon the focus error signal FES becomes greater than zero (FES>0). Conversely, when the optical disk is located at a distant position as shown in FIG. 15C, the vertical dimension of the spot 226 becomes longer, and the horizontal dimension of the same becomes shorter, whereupon the focus error signal FES becomes smaller than zero (FES<0). Focus control is performed in such a way that the focus error signal FES becomes equal to zero (FES=0) or comes to a predetermined value.
FIG. 16 is a cross-sectional view of an ordinary reflecting mirror and a Fresnel mirror 231. The Fresnel mirror 231 is a reflecting mirror that is made by cutting the ordinary reflecting mirror 230 in round slices along respective contour lines spaced apart from each other at a predetermined depth “d” and arranging the thus-cut round slices within a single thickness. Therefore, the Fresnel mirror 231 has a plurality of orbicular zone 232 and steps 233 that each connect adjacent orbicular zones 232. In the case of the Fresnel mirror 231, the zones 232 correspond to reflecting mirrors.
FIG. 17 is a plan view of an astigmatism generation element made up of a related-art Fresnel mirror. In FIG. 17, lines correspond to the steps 213, and areas located between the lines correspond to the zones 212. In the astigmatism generation element 206 made up of the Fresnel mirror 207, the steps 213 are made in the form of an inclined concentric oval shape.
Even when a Fresnel lens is used for the Fresnel mirror 207, an optical system of the optical pickup device, such as that mentioned above, can be built. JPS63-A-46402 shows an example using a Fresnel lens as an astigmatism generation element, and JP2008-A-90990 shows an example using a Fresnel mirror.
When the astigmatism generation element corresponds to a Fresnel mirror, the element is usually designed in such a way that a center portion of light reflected from the optical disk enters the innermost orbicular zone of the astigmatism generation element at an incident angle of 45° and exits at an exit angle of 45′ after undergoing reflection. However, the light reflected form the optical disk enters the astigmatism generation element in a spread manner. The reflected light enters an orbicular zone adjacent to a step located outside the innermost orbicular zone and an orbicular zone adjacent to a step located outside the second innermost orbicular zone, as well as entering the innermost orbicular zone. The step is a discontinuous area of the reflecting mirror, and incoming light enters the step at an inclination. Therefore, the light entered the step does not travel in a predetermined direction even after undergoing reflection and does not correctly enter the optical receiver. When resultant influence is great, a servo characteristic, which would originally be expected, cannot be exhibited in some cases.